Tripoli: Featuring armour-plated doors, secret passages and a state-of-the-art control room, the underground bunker of Mutassim Gaddafi, a son of the Libyan leader on the run, seems an impregnable citadel.

The sprawling facility is made up of 50 rooms buried under J'Raba Street in eastern Tripoli, protected by a double rampart.

"It was rumoured that it hid a nuclear facility and others said it was the house of Muammar Gaddafi himself," said Adel Tarbu, one of the rebel fighters who opened the big gate of the residence up to a group of journalists.

At first glance, it is a posh villa and its outbuilding is equipped with a swimming pool, gymnasium, sauna and bar. But its secret bunker is buried underground.

"Look at the thickness of the door," said the rebel fighter, pointing down a corridor to a room worthy of a bank vault and hooked up to its own ventilation system.

"Gaddafi described us as rats, but we see how his people lived underground as rodents," quipped Tarbu.

"We know the home of the British Prime Minister and the White House, and they are normal compared with the Gaddafi residences."

But he is adamant about the identity of the former occupant of the secret hideout. "We found papers and military documents of Mutassim and a photograph of one of his friends who was a model, a French woman," he added.

Born in 1975, Mutassim Gaddafi was trained by Egyptian officers and is a career soldier and doctor.

In 2007, his father promoted him to head of the National Security Council and in April 2009 he met US Secretary of State Hillary Clinton in Washington.

Suspected of attempting a coup, he was exiled to Egypt but was later pardoned and returned home. Before the conflict, he was seen as a rival to his older brother Seif al-Islam for the succession to his father.

The rebel fighter mocks the Gaddafi family's obsession with security, saying the facility "looks like a James Bond movie" as he pointed out yet more sophisticated surveillance equipment.

"We're right in Hollywood and yet this is Libya."

The bunker can be accessed by walkways to the garden outside.

Such passages are also found at the home of another Gaddafi son, Saadi, at a seaside cottage with bullet-proof glass over a tunnel that leads directly to a nearby avenue.

At their father's farm near Tripoli, the safety installations are as impressive with electrified fences, surveillance cameras and an electronic alarm system.